1. Field of the Invention
The present invention relates to a process for copolymerizing olefins at a high temperature of 130.degree. C. or above by the use of a novel Ziegler type catalyst system. More particularly, the invention relates to a process for producing an olefin copolymer having a narrow molecular weight distribution and a narrow composition distribution by the use of a solid catalyst component having a very high activity per transition metal.
2. Description of the Prior Art
The production of olefin polymers at high temperature by the use of Ziegler type catalyst is carried out according to the following processes. The first process is the so-called "solution process" in which an olefin is polymerized or copolymerized in the presence of a solvent such as cyclohexane or the like. In this process, an olefin is polymerized with a Ziegler type catalyst usually at a temperature of 120.degree. C. to 250.degree. C., at a pressure of 5 to 50 kg/cm.sup.2, in the state of polymer solution. The second process is the so-called "high-pressure ion process" in which an olefin is polymerized or copolymerized at a high temperature, at a high pressure, in the absence of solvent, in the state of molten polymer.
As are well known, these high-temperature solution polymerization process and high pressure ion polymerization process using Ziegler type catalyst are advantageous in that the reactor is compact and the comonomer can be selected with a high degree of freedom. In such high-temperature polymerization processes, however, many of the Ziegler type catalysts show a rapid decrease in the polymerization activity or catalyst efficiency in a relatively short period of time, even if they exhibit a high polymerization activity in the early stage of the polymerization, and they leave a large amount of catalyst residue after the polymerization. Particularly in case of transition metal catalysts such as Ziegler type catalyst, the catalyst residue remaining in polymer exercises an adverse influence on its quality, and therefore a large-scale installation such as a step for removing the catalyst or a step for purifying the polymer must be provided when a large amount of catalyst residue is left in polymer. In case that a metal halide such as halogenated titanium compound and the like is used as the solid catalyst, the apparatuses and instruments must be protected against the corrosion caused by active halogen, and therefore the polymerization activity per solid catalyst must be sufficiently high.
Now, olefin copolymers are put to very many uses such as film, laminate, coating of electric wire, injection molded articles, special molded articles, etc. As is generally known, the use of a polymer narrow in molecular weight distribution and composition distribution is desirable in these uses from the viewpoint of obtaining a product excellent in transparency, impact resistance, blocking resistance, etc. Particularly in case of copolymers, molecular weight distribution and composition distribution exercise an increasing influence upon the properties of olefin copolymer as the content of alpha-olefin in copolymer increases, so that an olefin copolymer having narrow molecular weight distribution and composition distribution is desired.
Although various improvements have hitherto been proposed concerning the Ziegler-type solid catalyst for use at high temperature (for example, Japanese patent application Kokai (Laid-Open) Nos. 51-144397, 54-52192, 56-18607, 56-99209, 57-87405, 57-153007, 57-190009 and 58-208303), none of them can be said to be satisfactory in the point of catalyst activity. Further, none of them can give a copolymer having narrow molecular weight distribution and composition distribution.
On the other hand, as a process for obtaining an olefin copolymer having narrow molecular weight distribution and composition distribution, there is known the method of copolymerizing an olefin by the use of a catalyst formed from a vanadium type catalyst component and an organoaluminum compound catalyst component. However, this catalyst is low in activity per transition metal, and its activity further decreases when used in copolymerization reaction at a high temperature of 130.degree. C. or above.